(i) Field of the Invention
The invention comes within the general scope of the reduction of the content of greenhouse gases in gaseous effluents of industrial origin discharged to the atmosphere. It is a question here of lowering nitrous oxide N2O in gaseous discharges.
For a long time, concern was only felt about the discharge of nitric oxides (NOx), which easily combine with water to form nitrous or nitric acids, the most spectacular sign of which is without doubt acid rain, with subsequent destruction of forests and damage to exposed monuments, and the most insidious signs of which are contamination of breathable air and its effect on public health. Awareness has now arisen of the significant contribution of nitrous oxide to enhancing the greenhouse effect, with the risk of leading to climatic changes with uncontrolled effects, and perhaps also of its participation in the destruction of the ozone layer. Its removal has thus become a preoccupation of the authorities and of manufacturers.
While the most significant sources of N2O are the oceans, uncultivated soils, agriculture, the combustion of organic matter and the use of fossil fuels, the chemical industry contributes some 5 to 10% of emissions of this gas. Nitric acid plants, as well as plants for organic synthesis employing nitric oxidation processes (production of adipic acid, of glyoxal, and the like), are the source of most discharges of N2O by the chemical industry (see, in this respect, Freek Kapteijn et al., Heterogenous Catalytic Decomposition of Nitrous Oxide, in Applied Catalysis B, Environmental 9, 1996, 25-64).
For some years already, most nitric acid plants have been equipped with so-called DeNOx, reactors, which operate satisfactorily in removing nitric oxides from their effluents. However, N2O which is essentially produced during the oxidation of ammonia over the platinum gauzes of the burners, remains substantially constant between the outlet of the burners and the inlet of the DeNOx, reactor and is not lowered by passage of the gases through this reactor (sometimes, it is even slightly increased).
Provision has been made to reduce the N2O content of the gaseous effluents resulting from nitric oxidation processes in organic chemistry by catalytically destroying the nitrous oxide contained in the latter over a mordenite/iron catalyst (EP 0,625,369). However, on account of the large fall in its activity in the presence of steam in the temperature range 350-450° C., this catalyst is not well suited to functioning with respect to dilute gases and ages badly, due to a mediocre hydrothermal resistance.
It also turns out to be economically unsuited to the treatment of the tail gases from nitric acid plants, which, upstream of the expansion turbine, generally correspond to the following characteristics,    temperature: <400° C.    N2O content: between 500 and 1500 ppmv,    NOx content: between 50 and 2000 ppmv,    H2O content: between 0.5 and 5%.
The economic optimization of the lowering of N2O both in the gases emitted by organic plants and by nitric acid plants involves the development of a catalyst which retains a good activity for the destruction of N2O at a temperature below 400° C. in the presence of NOx and of steam, and which has a sufficient hydrothermal stability at 600° C. to withstand the temperature peaks to which it may be subjected under certain circumstances in its use.
(ii) Description of Related Art
A solution corresponding to such specifications has just been found with a catalyst, composed of agglomerates formed of 80 to 90% of a ferrierite/iron assaying from 1 to 6% of iron, and preferably from 2 to 4%, and of 20 to 10% by weight of an agglomeration binder (percentages by weight with respect to the weight of the granule).